When the air conditioning load in the compartment of a car is decreased by an air conditioning system, or the temperature in the compartment of a car is below the predetermined temperature, the displacement of a compressor for an air conditioning system is generally too large, so that the compression ratio of the compressor can be decreased.
A scroll type compressor which can vary the compression ratio is well known, for example, U.S. Pat. No. 4,505,651 and commonly assigned copending application Ser. No. 669,389 filed on Nov. 8, 1984 show a scroll type compressor with a variable displacement mechanism.
However, in U.S. Pat. No. 4,505,651, the change of the compression ratio is not sufficient. Also, in the mechanism shown in U.S. patent application Ser. No. 669,389, the temperature of discharge fluid increases abnormally when the compressor operates at high speed.
One resolution to above problems is disclosed in commonly assigned copending U.S. patent application Ser. No. 875,561 filed on June 18, 1986.
Referring to FIG. 1 herein, a control mechanism 1 for a variable displacement mechanism as disclosed in U.S. patent application Ser. No. 875,561 is illustrated. Control mechanism 1 for varying the displacement of a compressor includes cylinder 2, piston 3 which is slidably disposed in cylinder 2 and spring 4 which is disposed between the bottom portion of cylinder 2 and piston 3. When electromagnetic valve 5 is energized, compressed gas in a discharge chamber (not shown) is introduced into the top of cylinder 2 through a capillary tube (not shown). Since the pressue of compressed gas is larger than the recoil strength of spring 4 and the pressure in intermediate pressure chamber 6, piston 3 is urged downward, and closes the opening from cylinder 2 to intermediate pressure chamber 6. Accordingly, the compression ratio of the compressor is increased.
On the other hand, when electromagnetic valve 5 is de-energized, compressed gas is not supplied to cylinder 2. Thus, the pressure in cylinder 2 becomes less than the recoil strength of spring 4 and the pressure in intermediate pressure chamber 6, piston 3 is urged upward, and intermediate pressure chamber 6 communicates with communicating chamber 7 through cylinder 2. Accordingly, refrigerant gas in intermediate pressure chamber 6 flows into communicating chamber 7 through cylinder 2. In this manner the volume of compressed gas which discharges into the discharge chamber is decreased, and the compression ratio of the compressor is decreased.
In a scroll type compressor with the abovementioned variable displacement mechanism, if electromagnetic valve 5 closes the communication between cylinder 2 and the discharge chamber in order to change the compression ratio from maximum to minimum, high pressure gas in the top of cylinder 2 gradually leaks into communicating chamber 7 and intermediate chamber 6 through a gap between the inner surface of cylinder 2 and the outer peripheral portions of piston 3. Thus, the pressure in cylinder 2 gradually decreases, and if it becomes less than the recoil strength of spring 4 and the pressure in intermediate pressure chamber 6, piston 3 is pushed upward. As a result of this upward movement of piston 3, the volume in the top of cylinder 2 decreases and the pressure in cylinder 2 increases. Therefore, piston 3 is urged downward again. However, since the compressed gas in the top of cylinder 2 continuously leaks into the communicating and intermediate chambers, piston 3 gradually moves up with vibrating motion. If vibration of the piston occurs continuously during operation, durability of the piston becomes a problem. Noise is also generated by the vibration of piston. Furthermore, stable operation of the variable displacement mechanism cannot be achieved when the piston vibrates within the cylinder.
Also, when piston 3 is urged downward and closes the opening of intermediate pressure chamber 6, the outer edge of piston 3 strikes against the inner bottom surface of cylinder 2 and causes impact noise. Also, piston 3 abrades due to the intermittent impact, adversely effecting the reliability of the control mechanism.